: This proposal aims to use the thoracic ganglion of the locust to examine the function of presynaptic inputs onto the central terminals of identified sensory neurons during the performance of several well-defined behaviors, in a mostly intact preparation. The goal is to search for general organizational principles relevant to the neuronal processing that underlies all rhythmic movements in all animals, and specifically rhythmic locomotory movements. Several hypotheses motivate the proposed experiments, including the ideas that (1) presynaptic inhibition to sensory terminals acts as a gain control mechanism to prevent saturation of the postsynaptic response, (2) presynaptic inhibition occurs preferentially during times when the sensory neurons are expected to be activated by self-generated movements, (3) similar presynaptic mechanisms operate in sensory-motor processing during other rhythmic movements besides walking, and (4) presynaptic influences onto sensory neurons are a convergent locus that is influenced by descending inputs as well as local feedback. These issues will be addressed by focussing on the following issues: (1) Do presynaptic inputs to sensory neurons relate their response properties to the phase of the step cycle during walking? (2) What presynaptic processing occurs in the sensory neuron terminals during ballistic movements of the leg? (3) Do descending signals from the brain also converge onto the central terminals of these sensory neurons? (4) Do different pathways that converge onto the same sensory neuron terminals act through different interneurons that use different neurotransmitters? (5) Can centrally generated and sensory feedback components of presynaptic inhibition during walking be dissociated and studied separately? (6) Do similar presynaptic events influence other sensory neurons during motor activity? (7) Can the inhibitory interneurons responsible for the presynaptic input be identified?